Direct converting transceivers are well known in the art. By “direct converting” it is meant that conversion is made directly from RF to baseband, without an intermediate frequency. Such direct converting transceivers may commonly be used in, for example, the radio of a mobile telephone, or the like. FIG. 1 illustrates the components of a typical direct converting transceiver which forms the basis of the present invention.
With reference to FIG. 1, here a baseband signal 10 is fed into the transmit path of the transceiver. The baseband signal 10 is filtered by a pulse shaping filter such as a root raised cosine (RRC) filter 12 so as to limit the bandwidth and hence limit spurious emissions. The output of the pulse shaping filter 12 is then input to an up-sampler 14, which increases the sampling rate from the typical four times oversampling of the baseband to provide a higher sampling rate for input to a transmit mixer 16. The transmit mixer 16 then modulates an RF carrier with the received up-sampled bit stream, to provide a modulated RF output signal, which is then input to a transmission amplifier 18. The transmission amplifier 18 is controlled by a transmission gain control signal, to apply a suitable signal gain to the modulated RF signal, for transmission. The amplified modulated RF signal is then input to duplexer 20, for feeding to antenna 21 for transmission.
With respect to the receiver signal path, antenna 21 receives modulated RF signals, which are fed via duplexer 20 into the receiver signal chain. The received signals are first amplified by low noise amplifier (LNA) 23, in accordance with an LNA gain control signal received by the low noise amplifier, and then demodulated from RF to baseband by receive mixer 22, which receives a suitable local oscillator signal. The thus obtained demodulated signals are then input to a receiver amplifier 24, which is controlled by a receiver amplifier gain control signal, in order to amplify the received signal by the required gain. The amplified and demodulated received signal is then input to anti-alias filter 26, to restrict the bandwidth of the signal prior to down sampling. The filtered received signal is then input to down sampler 28, which produces as baseband at four times oversampling rate, for processing by the remainder of the receiver signal chain. The down sampled received signal is then output to the rest of the receiver chain, as signal 30.
It will therefore be seen that the transmission path and the receiver path share the duplexer 20 in common, in order to be able to feed signals to and receive signals from the common antenna 21. In frequency division duplex systems where both the transmission chain and the receiver chain are active simultaneously, the problem of cross-talk between the transmission and receiver chain can therefore occur. In spite of chip planning and careful insulation between the transmitter chain and the receiver chain, the transmitted signal, due to its significantly higher signal power, can severely distort any received signal. The worst kind of distortion is clipping, where the transmitted signal saturates the transistors in the receiver chain. Clipping introduces non-linearities which may make restoration of the desired received signal impossible. Fortunately, clipping may be avoided by a properly designed automatic gain control (AGC) algorithm, and filtering.
In direct converting receivers particularly cross-talk is a problem, since here second order distortion introduced by the receive mixer 22 results in a cross-talk signal which is spectrally overlapping the receiver signal baseband, and thus is impossible to differentiate by spectral filtering, once introduced. The result from this kind of cross-talk is a decreased sensitivity due to the extra noise added by the cross-talk. Cross-talk in this way may be introduced either via poor on-chip transmitter and receiver path insulation, or from shared components such as the duplexer. Of these, the latter is believed to be the worst introducer of cross-talk into the receiver signal path, due to the power of the transmit signal present thereat.
Looking in closer detail at how cross-talk from the transmit signal can enter the receiver signal path, there are (at least) two ways for the transmit signal to enter the receiver path at baseband and thus distort the frequency range of the received, desired signal. It may be as an effect of inter-modulation distortion (IMD) in the receiver mixer in which case the transmit (TX) signal, XTX(f) (as a function of frequency), is part of either the received (RX) signal, XRX′(f) or the local oscillator (LO) signal, XLO′(f), in the mixer i.e.:XRX′(f)=XRX(f)+XTX(f) XLO′(f)=XLO(f)+XTX(f)
Higher order nonlinearities project the transmit signal onto baseband, here exemplified by the distorted XRX′(f). In particular, the output of the receiver mixer can be represented as:Y(f)=XRX′(f)XLO(f)+k2(XRX′(f)XLO(f))2+k3(XRX′(f)XLO(f))3+ . . .
Typically, the most significant of these terms is the second order inter-modulation distortion product (IMD2). Here, the square of the transmit signal TX results in:(XRX′(f)XLO(f))2=((XRX(f)+XTX(f))XLO(f))2=XTX2(f)XLO2(f)+ . . .
Here, only the interfering part of the TX signal is shown on the right-hand side; there will also be a second order RX signal, as well as a product of the RX, TX, and LO signals. In this respect, the second order IMD is mixed down into the frequency band of interest in the receiver, and hence creates cross-talk. Higher order IMD products are of course also created, but these are either too weak to pose a problem, or they get mixed to other spectral locations, and hence, whilst they may appear as cross-talk in the receiver signalling chain, they will typically be filtered out, for example by the anti-aliasing filter.
Another effect that introduces cross-talk into the receiver chain is by self-mixing where the transmit signal is found in both the RX signal and the LO signal, as shown:Y(f)=XRX′(f)XLO′(f)
First order mixing of the two then unavoidably results in cross-talk:Y(f)=(XRX(f)+XTX(f)(XLO(f))=XRX(f)XLO(f)+XRX(f)XTX(f)XLO(f)+XTX2(f)
Here, the first term is the wanted signal, the middle two terms will be filtered out, and the last term (the second order term) is the cross-talk.
Thus, as a result of both IMD in the RX mixer and self-mixing the transmit signal can find itself mixed into the band of interest to the receiver. Of course, in a time division duplex (TDD) system this is not of concern, as the receiver is not listening for a signal at the same time as the transmitter is transmitting. However, for frequency division duplex (FDD) systems, where the transmitter is transmitting at the same time as the receiver is listening (on a different frequency) then transmit signal cross-talk in the receiver signal chain can cause significant problems, resulting in the receiver failing to detect and successfully demodulate a signal being received. Moreover, because the cross-talk signal has been mixed into the region of the frequency spectrum of interest to the receiver, spectral filtering of the cross-talk signal cannot be performed without likewise filtering the received signal of interest.